The present invention relates to a mounting structure for an electronic device on which logical units and power supply units are to be mounted. More particularly, the invention relates to the mounting structure for an electronic device provided with such a power supply method and a unit layout as implementing the highly efficient power supply and cooling operation.
FIG. 7 shows an electronic device on which a multiple of printed boards are mounted according to the prior art. As shown in FIG. 7, for example, a host computer is configured so that logical units 22 and power supply units 23 are mounted on a frame 21 formed like a box. This type of computer includes a back board 25 one surface and the other surface of which normally have their corresponding units mounted thereon. In the configuration shown in FIG. 7, the logical units 22 are mounted on one surface of the back board 25 as being guided by the frame 21 and the guide rails 24. Each logical unit 22 includes lots of semiconductor elements (LSI) and the other components mounted thereon. In the lower portion of the box is located an air fan unit for feeding cooling air to the logical units 22 and the power supply units 23 located in the upper portion thereof. This cooling air serves to cool down the logical units 22 and the power supply units 23. The multiple of power supply units 23 operate to feed their respective powers. For example, as shown in FIG. 8, a power supply unit 23a feeds a power of 1.2 V, another power supply unit 23c feeds a power of 3.3 V, and another power supply unit 23c feeds a power of 5 V. In this case, the power supply unit 23a operates to feed a power of 1.2 V to the logical units 22 through a power supply bus bar 27 and the back board 25.
However, the conventional mounting structure includes set of logical units mounted collectively on one surface of the back board 25 and a set of power supply units 23 mounted collectively on the other surface thereof. This structure involves the following disadvantages. That is, since the logical units 22 generally have a larger heat quantity than the power supply units 23 and they are mounted in the upper and the lower stages of the frame 21, the higher heat value caused by the semiconductor elements included in the logical unit 22 is concentrated on one surface of the back board 25 (this side of FIG. 7).
As described above, one power supply unit 23a operates to feed a power of 1.2 V to a plurality of logical units 22 through the power supply bus bar 27 composing a power supply path 27a. Hence, the power supply bus bar 27 through which the power supply unit 23 feeds a power is connected to the back board 25 in a branched manner. It means that the length of the power supply bus bar 27 is respective depending on the distance between the power supply unit 23a and each logical unit 22. As a result, the shortest path and the longest path from the power supply unit 23 and the logical unit 22 may be provided. As the path is made longer, the drop of the voltage supplied from the power supply unit 23a is made larger. It has a problem of varying the voltage drop according to each path length.
Further, in the foregoing mounting structure, in a case that the two logical units 22 are mounted vertically in the upper and the lower stages, the guide rails 24 are located in the upper and the lower portions of the logical unit 22. However, where the logical unit is mounted only in the lower stage, the guide rail 24 for guiding the logical unit 22 located in the upper stage is not required, so that no guide rail 24 is located on the top surface of the box. As shown in FIG. 8, the guide rail 24 is a tabular form having an opening in the center thereof for holding the logical unit 22 on which the semiconductor elements are mounted. The provision of the guide rail 214 serves to narrow the flow path of the cooling air, thereby resisting the flow of the cooling air. Hence, if the guide rail 24 exists in some place while it does not exist in another place, the presence or the absence of the guide rail 24 for resisting the flow of the cooling air results in bringing about the variation of the quantity of the cooling air for cooling the logical unit 22.
Morever, as shown in Fig. 9, the foregoing mounting structure is constructed so that the logical unit 22 has plural LSIs 29 mounted thereon, the back board 25 has plural LSIs 28 as well, and the logical unit 22 is mounted on the back board 25 along the guide rail 30. This mounting structure is cooled by the cooling air fed from the lower portion of FIG. 9. However, the flow path of the cooling air for the LSIs 28 is not partitioned from the flow path of the cooling air whose temperature is raised by the heat of the LSIs 29. This phenomenon makes the design of the cooling structure difficult.